In optical discs, two-valued data are typically recorded into spiral or concentric tracks by various means; for example, convex-concave pits are formed by embossing into ROM discs, pores are formed into inorganic or organic recording membranes of recordable discs, and different crystal phases are formed into phase change discs. When recorded data are regenerated from optical discs, laser beams are irradiated to the tracks, the intensities of reflected light are detected, and regenerating signals are obtained. From the regenerating signals, two-valued data are generated through determining them by certain threshold values, for example.
The ROM discs such as CD, CD-ROM, and DVD-ROM discs are typically mass-produced inexpensively by transferring the convex-concave pits of stampers onto a resin plate. The convex-concave pits of stampers are typically produced, for example, by forming a resist pattern on a glass substrate, irradiating a laser beam onto the substrate to form a latent image, and developing the latent image.
Further, recordable discs or phase change discs such as CD-R, CD-RW, DVD-R, and DVD+RW are typically produced by transferring guide grooves for tracking etc. from stampers onto a resin plate, on which recording materials such as inorganic or organic recording membranes are applied, and information is recorded by means of reading-writing devices such as CD-R drives.
As for these optical discs or information recording media, there exist continuous needs to increase the recording capacity.
One way to increase the recording capacity of such information recording media is to decrease the size of guide grooves, pits, etc. However, focal diameter of laser beam typically depends on wavelength of the laser beam and numerical aperture (hereinafter referring to as “NA”) of the collecting lens; therefore, practical limits are considered as 405 nm of wavelength and 0.85 of NA, currently. For example, when NA is as high as 0.85, the distance between the pickup and the substrate becomes inevitably shorter, thus various troubles or adverse effects will occur due to collision of the pickup and the substrate or dusts, for example. For these reasons, NA of popular lenses utilized in DVD drives is usually 0.65, which will also exclude exchanges into higher NA from commercial reasons.
Another way to increase the recording capacity of such information recording media is to utilize multi-valued recording of three or more values in place of two-valued recording. For example, Patent Literature 1 discloses six-valued recording in one recording bit by altering the recording bit size of the phase change disc. FIG. 1 shows the model of the phase change disc. An image recording medium is prepared by forming recording material, reflective layer 5, and protective layers 1, 2, 3 on PC substrate 25 to which track 21 is formed. Laser beam is irradiated to the information recording medium to form recording bits 24. As shown in FIG. 1, recording bits 24 are is formed in a condition that one recording bit is formed per every certain area, as one cell for one recording bit, within groove 10 and the size of the recording bits are altered under certain rule. When the recorded information is regenerated from the image recording medium, the reflective optical quantities vary depending on the size of recording bits 24, RF signals are converted to levels in accordance with the reflective optical quantities, and information is regenerated. In such case, since values of 0 to 5 can be expressed by one cell, much information can be recorded compared to the conventional value of 0 and 1.
In order to increase the recording capacity by the multi-valued recording to a level higher than two-valued recording, the size of the recording pots should be decreased. When information of 25 GB is to be recorded by multi-valued recording in an optical disc of CD size, the length of unit cell is 250 nm or less and the diameter of the smallest pit is about 100 nm or less, in order to assure the dynamic range.
In general, stampers for producing optical discs are produced by exposing by means of a photoresist and laser beam, patterning of the resist by developing, and then etching of a substrate using the patterned resist as a mask. In the process for producing stampers, exposing by means of electron beam is a candidate for forming pits of about 100 nm in place of the laser beam.
However, resists are generally insufficient in sensitivity toward electron beam; and the processing capacity is inevitably lower since processing in vacuum atmosphere is necessary. Further, the utilization of electron beam suffers from hard maintenance, higher running cost compared to the laser beam process, and additionally higher process cost due to lower processing capacity.
On the other hand, the improvement of laser beam process is also expected for forming more fine patterns while avoiding possible higher cost described above.
Patent Literature 2 discloses a process for producing a stamper in which a laser beam is irradiated onto a phase change membrane such as of GeSn to crystallize the membrane in part, removing amorphous portions by etching, thereby forming a convex-concave pattern or structure.
Patent Literature 3 discloses a process for producing a stamper in which laser beam is irradiated to a chalcogen compound such as GeSbTeSn to crystallize the compound in part, removing amorphous portions by etching, thereby forming a convex-concave pattern or structure.
In order to achieve proper uniformity in structures having larger surface area such as optical discs, the remaining portions and the removing portions should represent significant difference between their etching rates or selectivities. The difference of etching rate or selectivity is generally not significant between crystalline and amorphous states in phase change materials. In some cases, an intermediate state between crystalline and amorphous appears in phase change materials. Accordingly, the processes disclosed in Patent Literatures 1 and 2 are considered inappropriate for achieving sufficient uniformity in structures having larger surface area.
Patent Literatures 4 and 5 disclose processes for producing a convex-concave pattern or structure by irradiating laser beam onto a thermosensitive material, which is formed by laminating two metal materials such as Al and Cu, thereby forming reacted portions of alloy through interdiffusion of two metal materials, and removing the unreacted portions by etching to produce a convex-concave pattern or structure.
Patent Literature 6 discloses a process for producing a convex-concave pattern or structure by irradiating laser beam onto a laminate of two inorganic materials such as Au and Sn, thereby forming reacted portions of alloy through interdiffusion of two metal materials, then removing the unreacted portions by etching to produce a convex-concave pattern or structure.
However, these processes are considered inappropriate for achieving sufficient uniformity in structures having larger surface area, since the distribution of two materials remains in the thickness direction, which tends to bring about a significant distribution in the thickness direction and a nonuniform etching rate.
Patent Literature 7 discloses a process for producing a convex-concave pattern or structure by irradiating laser beam onto a laminate formed of an optically absorptive and thermally exchangeable layer such as of GeSbTe and thermosensitive layer of chemically amplified resist for photolithography thereby to modify the thermosensitive layer, then removing the unmodified portions by etching to produce a convex-concave pattern or structure.
However, optically absorptive materials are generally inappropriate for producing convex-concave patterns or structures having a higher aspect ratio of pattern height to structure size, since the layer for the pattern or structure is necessarily thickened to obtain a higher aspect ratio, which leads to higher thermal diffusion in the layer and disturbance against fine and precise patterning.                Patent Literature 1: Japanese Patent No. 2642422        Patent Literature 2: Japanese Patent Application Laid-Open (JP-A) No. 09-115190        Patent Literature 3: JP-A No. 10-97738        Patent Literature 4: JP-A No. 2001-250279        Patent Literature 5: JP-A No. 2001-250280        Patent Literature 6: JP-A No. 2003-145941        Patent Literature 7: JP-A No. 2002-365806        